Metallocene catalyst systems are gaining importance as a new generation of catalyst systems for the preparation of polyolefins ("Single Site Catalysts"). These new catalysts essentially comprise, as already known from conventional Ziegler-Natta catalysts, a transition metal compound as the catalyst and a cocatalyst component, for example an alkylaluminoxane, in particular methylaluminoxane. Cyclopentadienyl, indenyl or fluorenyl derivatives of group IVa of the Periodic Table of the Elements are preferably employed as the transition metal compound. In contrast to conventional Ziegler-Natta catalysts, such systems not only have, in addition to a high activity and productivity, the capacity for targeted control of the product properties as a function of the components employed and the reaction conditions, but furthermore open up access to hitherto unknown polymer structures with promising properties with respect to industrial uses.
A large number of publications which relate to the preparation of specific polyolefins with such catalyst systems have appeared in the literature. A disadvantage in almost all cases, however, is the fact that a large excess of alkylaluminoxanes, based on the transition metal components, is necessary to achieve acceptable productivities (the ratio of aluminum in the form of the alkylaluminoxane to the transition metal is usually about 1000:1). Because of the high cost of the alkylaluminoxanes on the one hand and because of additional polymer working-up steps ("deashing steps") which are necessary in some cases on the other hand, polymer production on an industrial scale based on such catalyst systems would often be uneconomical. Furthermore, the solvent toluene which is often used for formulation of alkylaluminoxanes, in particular methylaluminoxane, is becoming increasingly undesirable for reasons of the storage stability of highly concentrated formulations (marked tendency of the aluminoxane solutions to form a gel) and for toxicological reasons with respect to the field of use of the polyolefins which finally result.
A significant reduction in the amount of alkylaluminoxane required with respect to the transition metal component can be achieved by applying the alkylaluminoxane to inert support materials, preferably SiO.sub.2 (J. C. W. Chien, D. He, J. Polym. Science Part A, Polym. Chem., Volume 29, 1603-1607 (1991). Such supported materials furthermore have the advantage of being easy to separate off in the case of polymerizations in a condensed phase (preparation of highly pure polymers) and of being usable as free-flowing powders in modern gas phase processes, in which the particle morphology of the polymer can be determined directly by the particle form of the support. Alkylaluminoxanes fixed on a support are furthermore physically more stable, as dry powders, than solutions of comparable Al content. This applies in particular to methylaluminoxane, which, as already mentioned, tends to form a gel in solution in toluene after a certain storage time.
Metallocene catalyst systems, too, or precisely those formed from the aluminoxane with the metallocenes, are considerably more stable in supported form than in solution.
Some possibilities for fixing alkylaluminoxanes to supports are already described in the literature:
EP-A-O 369 675 (Exxon Chemical) describes a process in which immobilization of alkylaluminoxanes by reaction of an approximately 10% strength solution of trialkylaluminum in heptane with hydrated silica (8.7% by weight of H.sub.2 O) is achieved.
In EP-A-O 442 725 (Mitsui Petrochemical), the immobilization is effected by reaction of a toluene/water emulsion with an approximately 7% strength solution of trialkylaluminum in toluene in the presence of silica at temperatures of -50.degree. C. to +80.degree. C.
U.S. Pat. No. 5,026,797 (Mitsubishi Petrochemical) opens up another alternative by reaction of already pre-prepared alkylaluminoxane solutions with silica (predried at 600.degree. C.) at 60.degree. C. and subsequent washing out of the non-immobilized alkylaluminoxane content with toluene.
Finally, U.S. Pat. No. 4,921,825 (Mitsui Petrochemical) describes a process for immobilizing alkylaluminoxane by precipitation from solutions in toluene by means of n-decane in the presence of silica.
Some of these processes are technically involved, since, inter alia, they include low reaction temperatures at the start or multi-stage working-up processes and, as a result, losses in yield in respect of the amount of aluminum employed in the form of aluminum trialkyls. Furthermore, the space/time yield is sometimes impaired considerably by the obligatory use of relatively large amounts of solvent.
Finally, the metallocene must also subsequently be fixed to the support in order to obtain an active polymerization catalyst. A further reaction step in a solvent is therefore necessary. As a result, the profitability of these systems is jeopardized once more.
Several possibilities likewise exist for fixing the metallocenes to the support.
Fixing to the support is understood as meaning, according to the definitions, immobilizing of the component employed on an inert support; the components are deposited on the support material in a manner such that subsequent elution by solvents is no longer possible.
Thus, on the one hand, the metallocene can be brought into contact from solution with the suspended supported aluminoxane, or the metallocene can first be reacted with the aluminoxane and the reaction product can subsequently be applied to the inert support. With both methods, the working-up steps are not trivial, since the success of the application to the support and the activity of the finished catalyst depend decisively on the reaction temperatures and the drying conditions (cf. EP-A-O 560 128, U.S. Pat. No. 5,308,815).
An object of the present invention is therefore to overcome these disadvantages of the prior art and to provide an economical process by means of which active catalysts for olefin polymerization comprising alkylaluminoxanes and metallocenes can be fixed to inert support materials in one process step, largely without the co-use of organic solvents, in a high yield and homogeneity and in a reproducible manner, the particle morphology of the support being retained and the products finally being in the form of free-flowing powders.
According to EP-A-O 672 671, it has been found that some of the disadvantages mentioned above can be eliminated by carrying out the synthesis of alkylaluminoxanes, in particular methylaluminoxanes (MAO) and fixing thereof to inert supports directly via the gas phase without any use of solvents and without additional process steps.
It has now been found that here also the metallocene can be supported in the gas phase simultaneously and together with or after the aluminoxane by varying the geometry of the unit, regardless of whether the aluminoxane has been prepared in the gas phase or according to the prior art after fixing to the support in the liquid phase and subsequent drying out conventionally or according to the invention.
In one variant of the procedure according to the invention, prepolymerization in one step is also possible, so that the end product is available immediately for the polymerization.
The resulting end products are free-flowing powders which can be employed directly as highly active catalysts for olefin polymerization. The particle morphology and particle size distribution are changed not adversely but rather positively within the process. The fines content of the support material can be built up by the application to the support within the gas phase, and on the other hand extremely small particles can also be removed in a controlled manner. By varying the gas streams and using suitable reactor types and reactor geometries, selected discharge of the product oriented to particle size is also possible. The particle size, the activity and the catalyst concentration can thus be adjusted by controlled adjustment of the operating parameters.